Various systems, hereinafter referred to as “tracker systems”, and methods, hereinafter “wireless location technologies” (WILOTs), for wireless determination of a location of a mobile transmitter and/or receiver terminal and a person or object, a “bearer”, carrying or mounted with the mobile terminal, are known. Common transmitter and/or receiver terminals that incorporate and/or are located by WILOT tracker systems are mobile phones, global positioning satellite (GPS) receivers, computers, personal data assistants (PDAs), and workbooks. Among common wireless location technologies employed by tracker systems to determine a location of a mobile transmitter and/or receiver and thereby its bearer, are technologies referred to as trilateration and multilateration. Hereinafter, a transmitter and/or receiver, and/or a device in which it is housed, and/or its bearer, that are located by a tracker system are distinguished as being a “target”, used as a modifier or noun, of the tracker system.
In some WILOT tracker systems using trilateration location technologies and apparatus, signals from three or four transmitters having known locations are received by a target receiver and used to determine a transit time from each transmitter to the target receiver. Each transit time defines a spherical surface having its center at the transmitter for which the transit time was determined and a radius equal to the speed of light times the transit time. Were all the transit times and locations of the transmitters known with absolute accuracy, the spherical surfaces would have a well defined common intersection point, at which the target receiver would be located.
In practice of course, the transit times and transmitter locations are not known with absolute accuracy, and the spherical surfaces in general do not intersect at a well defined common point. The target receiver (and thereby its bearer), is therefore generally determined to be located in a region of uncertainty (ROU) in which all the spheres come closest to intersecting. A size of the ROU and therefore accuracy of location is dependent on, among other factors, accuracy of synchronization of clocks in the transmitters and receiver that are used to determine transit times between the transmitters and the receiver.
Size of an ROU associated with a location of a target is assumed to be determined by a characteristic linear dimension, such as a radius or diameter, of an area of uncertainty associated with the location. An ROU having a characteristic dimension “X”, in given units of length may be recited as an “ROU of X” in the given units. An ROU, unless otherwise specified is assumed to have a centroid coincident with a target location with which it is associated. For a circular ROU the center of the ROU circle is coincident with the target location. An ROU, unless otherwise specified, is considered to have a spatial location defined by a target location with which it is associated and reference to the ROU is considered to include a reference to its associated target location.
In tracker systems that use the GPS system, GPS receivers, such as are commonly available for locating vehicles and persons, are located by trilateration using clock signals transmitted from at least three GPS satellites. GPS satellite clock signals are generally accurate to ±200 ns (nanoseconds) relative to Universal Time Coordinated (UTC) and some GPS systems can locate a receiver in an ROU having a characteristic dimension of a few tens of centimeters.
A trilateration technology may of course be used in “reverse”, with a single transmitter transmitting signals to three or four receivers to provide signal transit times useable to determine a location of the transmitter.
In WILOT tracker systems using multilateration location technologies and apparatus, time differences of arrival, or differences in signal strength, of signals from three or more synchronized transmitters received at a receiver are used to determine location of the receiver. Mobile phone networks may use multilateration location systems in which synchronized base station transmitters from different cells in the network transmit signals to mobile terminals, such as mobile phones, personal digital assistants (PDA), and laptop computers, to provide locations for the mobile terminals.
Accuracy of positioning provided by a mobile phone network multilateration technology is generally less than accuracy of location provided by GPS based trilateration technologies. Accuracy may be influenced by size of the cells in the mobile phone network, which may have characteristic dimension that range from about 100 m (meters) to about 3 km (kilometers). Usually, a mobile phone network provides locations having ROUs of dimensions between about 1,000 m (meters) and about 2,000 m.
As in trilateration location technologies, multilateration location technologies may be operated in “reverse”, with differences between times of arrival or signal strengths of signals from a single transmitter received at three or four receivers being used to determine location of the single transmitter.
Many mobile terminals are now equipped with inertial navigators. An inertial navigator typically comprises a set of accelerometers and gyroscopes and integrates measurements of acceleration provided by the accelerometers and gyroscopes to “dead reckon” a path traveled by the navigator from a starting location. A terminal point of the integrated path provides a location of the navigator and the navigator's bearer relative to the starting location. Whereas an inertial navigator operates differently than the examples of WILOT systems discussed above, an inertial navigator is considered a WILOT tracker and is distinguished from other WILOT trackers when its differences from other WILOT trackers are pertinent to the discussion.
Errors in a location provided by an inertial navigator propagate and tend to increase as time over which a path is integrated and length of the integrated path increases. Inexpensive accelerometers and gyroscopes comprised in a consumer inertial navigator suffer from drift that degrades accuracy of location provided by the navigator relatively rapidly with integration time and/or path length of a path the navigator integrates. As a result, ROUs for locations determined by commercial inertial navigators may have characteristic dimensions that grow to hundreds of meters over a dead reckoning integration period of about a half hour.
Whereas GPS based tracker systems generally provide the most accurate determinations of locations, they require relatively large amounts of power, and generally do not function at locations for which line of sight to at least three GPS satellites is not available. Various multilateral and trilateral tracker systems are subject to disturbance by multipath signaling, in which energy from a same signal travels by more than one path to a target receiver, arrives at the receiver at different times, and degrades measurements of signal transit times and/or signal strengths. Accuracy of both trilateral and multilateral tracker systems is compromised by loss or degradation of synchronization between clocks in the systems. As a result, the various WILOT tracker systems often become erratic and provide locations for a target that are unreliable.